This invention relates generally to stabilizing the light energy output of a laser diode, and more particularly to a method and apparatus for precisely controlling laser diode output energy as the laser is pulsed on and off.
A laser diode is a semiconductor device whose purpose is to provide, as much as possible, coherent light of a specific wavelength and power level. This device has a PN junction similar to any diode device. The differences among semiconductor diodes are in the materials used to create and dope the diode, as well as the PN junction structure. In a laser diode, the junction structure is used to provide a light channel with mirrored ends to produce a resonant cavity for the correct output frequency, that is, for the desired lasing wavelength.
Laser diodes are commonly used in many image scanning applications, and in particular, are used for exposing film or other light sensitive media as the output of the laser diode is scanned across an image line. In a typical application, after each line is scanned, the medium is moved in a direction transverse to the scan direction for sequentially exposing a series of lines on the medium. The result is an image which can be used in any of a number of applications, including, for example, half-tone presentations.
In operation, as in any laser, the electrons are pumped into a higher energy level to produce lasing action. This action creates a population inversion. The pumping action in the laser diode is accomplished by injecting a current into the PN junction; and therefore, for lasing action to occur, the laser diode must be forward biased.
When the laser is forward biased it will emit energy at the appropriate wavelength. However, as the diode begins to heat, the lasing action becomes less efficient, and the emitted power level will decrease in an exponential manner. This decrease in power level is known as "droop" in the industry, and many manufacturers of laser diodes specify this parameter. Thus, since the output of the laser diode varies in accordance with its junction temperature, a laser diode starting "cold" will have a different initial output power for a given input drive current than a laser diode which has recently been used and has a junction starting at an elevated temperature. Thus it is known, that for a selected laser input current, the light energy output decreases with increasing junction temperature.
In a precision laser scanning application, "droop" is of particular significance because it will show up as a tonal variation in the output copy of the image being produced. A known method for dealing with this phenomenon includes maintaining the laser diode at a level of lasing (when it is supposedly "off") that would reduce the droop effect by keeping the junction temperature high. However, this method will not compensate for the total effect and has the added disadvantage of film fogging and/or the requirement of adding a neutral density filter.
Another method controls the actual picture element (pel). This is done by controlling the laser diode for each picture element produced. For high speeds and high precision, this turns out to be a costly approach.
Accordingly, it is an object of the invention to stabilize the laser diode output energy as a function of current input using inexpensive, stable circuit components and without the use of additional filters. Another object of the invention is to compensate for variations in the temperature of the laser diode junction.